The present application claims priority to Japanese Patent Application No.11-80346 filed Mar. 24, 1999, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to a sheet decurling apparatus, and more particularly to a sheet decurling apparatus that corrects the curling of sheets of paper caused by the thermal fixing of the toner image onto the sheet in image forming apparatuses such as copying machines, printers and facsimile machines.
2. Description of the Related Art
The conventionally known sheet decurling apparatus of this type has a decurling conveyance path in which decurling takes place and a non-decurling conveyance path in which decurling does not take place, such that curled sheets are automatically guided to the decurling conveyance path based on the amount of curling in the front end of the sheet, and sheets that are not curled are guided to the non-decurling conveyance path by a fixed conveyance path alternating device, as disclosed in U.S. Pat. No. 5,300,012.
Other technologies have also been proposed in which the amount and direction of curling in the sheet is detected by a sensor and an appropriate conveyance path is selected accordingly, or in which the device is equipped with a decurling mechanism that allows the degree of decurling to be adjusted.
As a method to decurl, as disclosed in Japanese Laid-Open Patent Application Hei 4-173655, a technology has been proposed to decurl by forming a loop in the sheet in the opposite direction from the curling based on a difference in speed between the rollers.
However, in the method disclosed in U.S. Pat. No. 5,300,012, because the selection of a conveyance path depends on the amount of curling in the front end of the sheet, a non-curled sheet may be sent to the correction path by mistake, depending on the conveyance situation. In such a case, a sheet that does not require curling is curled unnecessarily.
In contrast, the technology disclosed in Japanese Laid-Open Patent Application provides an appropriate correction effect based on the detection of the amount of curling. However, because the amount of curling in the front end of the sheet is detected by multiple transmission sensors, erroneous detection is quite likely due to the behavior of the sheet during conveyance, and therefore the technology is impractical. Furthermore, the technology has the reverse effect that, because the decurling device is set to appropriately correct the curling of thin sheets, which are the main type of paper used, when a thick sheet of paper is conveyed, substantial curling occurs in the opposite direction from normal curling.
The object of the present invention is to provide an image forming apparatus that can appropriately decurl in thin sheets of paper and does not cause curling in thick sheets of paper in the opposite direction from normal curling.
Another object of the present invention is to provide a sheet decurling apparatus having a simple construction that automatically cancels the decurling operation for sheets in which curling is not likely, such as thick sheets, and that reliably corrects thin paper with high moisture content, where large curling tends to occur.
These and other objects are attained by a sheet decurling apparatus comprising a first conveyance device that conveys the sheet; a second conveyance device that is located downstream from the first conveyance device and that coveys the sheet further downstream; a drive source to drive the first and second conveyance device; a control device that controls the speed of the first and second conveyance device to form a loop in the sheet between the first and second conveyance device; and a space that is located between the first and second conveyance device and that receives the loop in the sheet, wherein when the sheet""s rigidity exceeds a prescribed level, slipping occurs between the drive source and the sheet.
In the construction described above, where curling is not likely, such as with rigid sheets such as thick paper or paper with low moisture content, slipping occurs in the drive path due to the rigidity of the sheet itself. Therefore, loop formation is automatically cancelled, and consequently no unnecessary curling occurs. Thin paper with a high moisture content that is highly subject to curling is not rigid, and therefore no slipping occurs in the drive path, such that a loop is formed in the opposite direction from the curling, leading to reliable correction of the curling. In order to form a loop in a sheet, any method may be used, including a method in which the conveyance by the downstream second conveyance device is stopped for a prescribed period of time.
If control is performed such that the second conveyance device conveys the sheet at a slower speed than the first conveyance device, the loop moves from the front end to the trailing edge of the sheet being conveyed, and therefore a decurling effect may be obtained for the entire sheet. If the difference in conveyance speed between the first conveyance device and the second conveyance device is maintained, the loop continues to grow, which increases the decurling effect on the trailing edge of the sheet.
In addition, it is preferred that a drive force limiting device be used that can employ a different torque depending on the direction of conveyance, such as a torque limiter or one-way clutch, so that slipping will occur between the drive source and the sheet when the sheet""s rigidity exceeds a prescribed level. If a drive force limiting device is used, a difference in conveyance drive force may be reliably generated so that a loop having an appropriate size may be automatically formed in accordance with the rigidity of the sheet.
Where a drive force limiting device having a lower torque for the direction of conveyance than for the opposite direction is mounted to the first conveyance device, if a rigid sheet is used that does not need much decurling, such as thick paper, the drive force limiting device for the first conveyance device spins idly and the drive is not transmitted. This situation creates the same effect as the slipping of the sheet, making conveyance impossible, and as a result no unnecessary loop is formed.
Where a drive force limiting device having a higher torque for the direction of conveyance than for the opposite direction is mounted to the second conveyance device, when the sheet is conveyed by the drive force of the second conveyance device, the sheet moves faster than the drive speed, resulting in cancellation of the loop.
If the sheet decurling apparatus is located downstream from the fixing device of the image forming apparatus, decurling may be effectively performed to the sheet that has just undergone fixing and been curled. If the fixing device of the image forming apparatus is used as the first conveyance device, no special conveyance device need be added, and an increase in cost may thereby be avoided.
Where the fixing device comprises a pair of heat rollers, the space to receive the loop of the sheet is created on the side of the roller having the lower temperature, relative to the sheet conveyance path. In this construction, while the curling that occurs in the sheet through the thermal fixing process is generally oriented toward the side of the element of the thermal fixing device having the higher temperature, a loop is formed in the opposite direction from the curling, whereby decurling may be effective performed.
The first conveyance device may be located inside the finisher connected to the image forming apparatus. Alternatively, the first conveyance device may be located on the side of the image forming apparatus, while the second conveyance device may be located on the side of the sheet processing device connected to the image forming apparatus. In this construction, because a connecting area between the two devices is used, a compact apparatus having a decurling effect on curling may be obtained.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.